1. Field of the Invention
The present invention relates to a titanium alloy sheet which has excellent workability and small anisotropy with respect to mechanical properties such as bending properties and a production method thereof.
2. Description of Related Art
Generally, a titanium alloy sheet is produced as a coil by hot rolling that uses a tandem mill, and such a coil is cut to a predetermined length for sheet forming. With respect to titanium alloys that have relatively poor workability such as .alpha.+.beta. type titanium alloys, a so-called "pack rolling" is performed, mainly using a reverse mill, in which a titanium alloy slab is packed with carbon steels, for example, by covering the upper and down sides of a slab with carbon steels or by inserting a slab in a carbon steel box before rolling, so that a decrease in temperature during rolling is suppressed and rolling is performed in a high temperature range where the titanium alloy has relatively good workability.
In any one of the methods described above, hot rolling is performed in the air, and then, oxide scales which are formed on surfaces during heating or rolling and oxygen-enriched layers underneath are removed in a grinding process, for example, using a coil grinder or a sheet grinder.
It is an essential step for improving the quality of a titanium alloy sheet to remove oxide scales on surfaces and oxygen-enriched layers underneath. The reason is that, if oxide scales and oxygen-enriched layers remain, the appearance of a product deteriorates, and since the areas near the surfaces are significantly hardened by the oxide scales and the oxygen-enriched layers, the workability such as bending properties' deteriorates.
The workability, such as bending properties, is sensitively influenced by surface conditions such as surface roughness, and thus, in a grinding process, a grindstone, an abrasive belt and abrasive grains are appropriately combined in order to control the surface roughness of the finish.
However, since grinding is performed in one direction, there is a difference in surface conditions such as surface roughness between the grinding direction and a direction transversal to it. Thereby, the workability is good in the grinding direction, however, significantly bad in the direction transversal to the grinding direction, resulting in a large anisotropy with respect to mechanical properties.
In actually forming a titanium alloy sheet into a certain component, a step of bending or the like is performed, and the bending direction is not limited to the grinding direction. The bending may be performed in any direction, for example, in the direction transversal or diagonal to the grinding direction. Although excellent workability is desired in all directions of a sheet plane, the current titanium alloy sheet which is ground in one direction cannot meet this requirement.